feat: export error codes as static properties

[alchar]

This is just to make it possible to get the code, without having to create a new instance of the class.

In our use case, we emit a metric with the error code on some failures. The unit tests look like:

assert(emittedMetric === new JWTExpired("test message").code);

which isn't nearly as clean as:

assert(emittedMetric === JWTExpired.code);

Discussion is here